1. Field of Invention
The invention relates to a method of arranging orifices on a print head for inkjet printers and the corresponding structure.
2. Related Art
Conventional inkjet printers use color mixing or half-tone processing of ink droplets to print colors at desired pixel positions on a medium (such as paper or transparencies), producing varied color levels thereon. However, the sizes of conventional ink droplets are slightly bigger, roughly 80-100 pl. (pico-liter). Taking the printing resolution of 300 dpi (dot per inch) as an example, a single droplet occupies a whole pixel. Therefore, there are very limited possible color variations.
To enhance the printing quality, producing more continuous, varied colors, the key issue is to make each pixel contain as many colors as possible. A trivial solution is to make the ink droplets smaller. For example, the photo resolution enhancement technology (PhotoRET) proposed by Hewlett Packard is one of the solutions to enhance the photo image quality. On the other hand, color print heads (with CMY colors) are also improved so that each droplet jetted out is only 30 pl. big. Therefore, any pixel position can be controlled to have more droplets (0-3 droplets). As the number of droplets in each pixel changes, the area occupied also varies to present different colors. The ink droplet jetted out of the state-of-art print heads can be even smaller, about 10 pl. That is, each pixel can have 0-8 ink droplets. As shown in FIGS. 1A, 1B, 1C, and 1D (xe2x80x9cPhotographic Quality Imaging With HP Thermal Inkjetxe2x80x9d, Edited by Dr. Ross R Allen, Printing Technology Department HP Laboratories, Palo Alto, Calif. USA.), if the print head 10 prints none, 1 droplet, three droplets or 8 droplets of ink 20, respectively, the medium 30 will have four color levels ranging from white to dark. With color mixing effect, more than 250 colors can be produced on the medium, greatly enriching the color contents and increasing the number of color levels.
This type of multilayer dot printing can provide photograph quality printing. The increase in color levels does not need half-tone processing when the image color changes, rendering smoother gradient fill. Furthermore various printing color combinations allow the fine-tuning function of printers for color processing and corrections.
When applying the above-mentioned multilayer dot printing in light color areas, the number of ink droplets 20 needed is less (please refer to the area being colored with ink droplets 20 in FIGS. 1B and 1C are that in FIG. 3 with 0-7 ink droplets) and the ink droplets are sprayed at the same position on the medium 30. Therefore, the printed area becomes bigger and bigger but it never fills the whole pixel, resulting in gaps that are not covered by colors among pixels, as shown in FIG. 2 (xe2x80x9cPhotographic Quality Imaging With HP Thermal Inkjetxe2x80x9d, Edited by Dr. Ross R Allen, Printing Technology Department HP Laboratories, Palo Alto, Calif. USA.). Taking a sheet of white paper as an example, white spots 40 are formed after printing, seriously deteriorating the image quality. Moreover, as one can see from FIG. 4, when the number of ink droplets is 4, the open lines 50 produced in the horizontal and vertical directions can be easily discovered with even naked eyes. So the multilayer dot technology still has many defects for many specific image contents, far from reaching the photo image quality.
An object of the invention is to provide a method of arranging orifices on a print head and the corresponding structure so that the sizes of open lines can be minimized in printing.
According to the disclosed method and structure, the invention properly arranges relative positions of orifices on a print head so that diff rent ink droplets for the same pixel fall at different positions in the pixel. The distribution of the ink droplets in the pixel can be more homogeneous so that no obvious open lines can be seen.